The long-term objective of this research program is to understand the roles of key coagulation and fibrinolytic factors in inflammatory host defense and bacterial virulence. Since a rigorous understanding of these complex processes can only be achieved in an in vivo experimental setting, our proposed studies focus on innate immune surveillance and bacterial pathogenesis in gene-targeted mice with specific alterations in core hemostatic factors. The aims of this project center on the following specific hypotheses: i) host factors mediating fibrin deposition and dissolution are important determinants of the inflammatory response in vivo, ii) bacterial factors known to bind/activate host prothrombin, fibrinogen and plasminogen serve to subvert host defense, iii) leukocyte engagement of fibrin within challenged tissues is an important cue in "target recognition" leading to the implementation of effective antimicrobial functions, and iv) hemostatic factors control leukocyte activation events as well as macrophage egress from inflammatory sites into lymphatics. These hypotheses will be tested through detailed studies of S. aureus virulence/host defense in mice lacking fibrin(ogen) or expressing mutant forms of fibrinogen that either lack the leukocyte integrin receptor alphaMbeta2 binding motif, lack platelet integrin receptor alpha(IIb)beta3 binding motif, or cannot be converted to a fibrin matrix (Aim I). Further, the role of procoagulant and fibrinolytic factors in establishing S. aureus virulence/host defense will be determined through investigator-imposed genetic changes in prothrombin, plasminogen and their respective microbial-derived activators: coagulase and staphylokinase (Aim 2 and 3). Finally, the mechanistic role of neutrophils and macrophages in the fibrinogen/prothrombin-dependent bacterial clearance will be established in the context of S. aureus peritonitis (Aim 3). The proposed studies will provide a detailed understanding of the cross-talk between the hemostatic and inflammatory systems in the implementation of effective antimicrobial response in vivo. Further, these studies may illuminate therapeutic strategies centering on specific hemostatic system components that are effective in the treatment of life-threatening microbial infection/sepsis. Lay Description: The bacterial species that are the most common causes of human suffering and death have uniformly evolved the means to engage coagulation factors, apparently as a means of subverting host defense. The goal of this research is to develop a detailed mechanistic understanding of the role of host hemostatic factors in bacterial virulence and the inflammatory response as a step toward novel therapeutic interventions for life-threatening infectious disease. [unreadable] [unreadable] [unreadable]